fix(test): use invoke_group in test_multiple_invokes to avoid race

With invoke=[task_a, task_b], each task is a separate invocation that
sends its own done.invoke — the first one triggers the transition and
cancels the second. Using invoke_group ensures both complete before a
single done.invoke is sent.

Author: fgmacedo

tests/test_invoke.py

@@ -254,7 +254,7 @@ def task_b():
             return "b"
 
         class SM(StateChart):
-            loading = State(initial=True, invoke=[task_a, task_b])
+            loading = State(initial=True, invoke=invoke_group(task_a, task_b))
             ready = State(final=True)
             done_invoke_loading = loading.to(ready)